The site consists of a small D-shaped enclosure, its boundaries marked by crumbling walls of mortared masonry that follow the natural contours of the hill. Though much reduced over the centuries, the defensive wall shows signs of sophisticated construction, including the remnants of an external batter at its base; a sloped reinforcement typical of medieval fortifications. The northern section stands as the best preserved portion, whilst elsewhere the structure has largely collapsed into grass-covered rubble, with only the wall footings visible beneath the turf.

A gap in the wall on the north-northeast side, measuring just over two metres wide, marks what was likely the main entrance to the enclosure. From this opening, the faint traces of an avenue can still be detected, defined by two parallel banks of earth and stone that lead down the hillside; perhaps the original approach road used by those who once occupied this defensive site. Inside the enclosure, against the northern wall, lie the stone foundations of what appears to be an internal structure, though its exact purpose remains unclear.

Local tradition, recorded by the Ordnance Survey, holds that this townland once contained ‘the remains of an old castle, which was built by the Brennan family’. Whilst the exact location of the Brennan castle isn’t definitively established, these ruins on the hilltop are quite possibly the structure mentioned in local lore. The confusion may stem from the fact that Killinlahan sits adjacent to the townland of Ballybrennan, and the castle associated with the Brennan name could potentially have stood in either location. Whether this was indeed the Brennan stronghold or another fortified site altogether, the ruins offer a tangible link to the medieval families who once controlled these lands.